1. Field of the Invention
The present invention relates to an optical printer head to expose a photosensitive body in a xerography-type printer.
2. Description of the Related Art
Conventionally, as a xerography-type printer, a laser printer and/or line-illuminant type optical printer are known. The laser printer produces printed outputs by causing modulated laser light generated through modulation of laser light based on data to be output to scan on a photosensitive drum, using two or more lens systems and polygon mirrors, to expose an image and to develop it. Since the laser printer can provide a higher speed, higher image quality and lower noise when compared with a dot-impact type printer or ink-jet type printer or a like and allows printing on ordinary paper, it is not only widely used for business applications but also is finding widespread use in households in recent years.
The line-illuminant type optical printer is a printer which uses a line illuminant composed of light emitting devices arranged in a form of a line. Since each of the arranged light emitting devices irradiates a corresponding spot on a photosensitive body, it has an advantage in that a scanning optical system is not required, which serves to implement high reliability and miniaturization of optical printers.
FIG. 13 is a side view showing overall configurations of conventional optical printer using a line illuminant. The conventional optical printer using the line illuminant, as shown in FIG. 13, is chiefly composed of a data inputting device 101, an optical printer head 102, a converging rod lens array 103, a photosensitive drum 104, a charging device 105, a developing device 106, a transferring device 107, a charge removing device 108 and a cleaning device 109.
Operations of the optical printer using the line illuminant will be hereinafter described by referring to FIG. 13.
Print data fed from the data inputting device 101 is input to a driving circuit (not shown) of the optical printer head 102. An output from the driving circuit activates the optical printer head 102 to cause the line illuminant to emit light. The photosensitive drum 104 is irradiated with light emitted by the activation of the optical printer head 102 and converged by the converging rod lens array 103. A surface of the photosensitive drum 104 is uniformly pre-charged by the charging device 105. A charge is removed from an area of the surface irradiated with the light by the optical printer head 102 and an electrostatic latent image is written to the photosensitive drum 104. Electrically charged fine grains (toner) are deposited on a surface of the photosensitive drum 104 to which the electrostatic latent image has been written and the electrostatic latent image is developed by the developing device 106 and a toner image is generated. The toner image having reached an object to be printed 110 through rotation of the photosensitive drum 104 is transferred to the object to be printed 110 by electric fields applied from the transferring device 107 and then a transferred toner image is fixed on the object to be printed 110 by a fixing device (not shown). Residual charge being left on the surface of the photosensitive drum 104 after it has passed through the transferring device 107 is removed by the charge removing device 108 and finally the toner being left on the surface of the photosensitive drum 104 after it has been transferred is removed by the cleaning device 109.
Moreover, a line-illuminant type optical printer head is disclosed in Japanese Patent Application Laid-open No. Sho58-65682 in which an LED (Light Emitting Diode) arranged in a form of a line is used as a line illuminant. An LED-type optical printer head uses a ceramic substrate made from alumina on which LED chips are arranged in the form of a line. IC (Integrated Circuit) chips operating as driving circuits are die-bonded to both sides of the ceramic substrate by using a conductive paste and then electrical connections are established by wire bonding and electrical signals and power are supplied through a FPC (Flexible Printing Cable) from a printer proper to the ceramic substrate. Due to size limitation imposed by an n-type GaAsP (Galium Arsenide Phosphide) substrate and to yield limitation imposed by manufacturing processes, LED chips of 64 dots or 128 dots with about 60 xcexcm pitches are used presently. Therefore, to form the line illuminant of the optical printer head, it is necessary to arrange a plurality of such LED chips and, if accuracy of arrangement must be increased, cutting and/or mounting technologies with a high accuracy in the order of microns are required. Moreover, the n-type GaAsP substrate is small and costly and further has many defects. That is, in a case of monolithic type LED chips, an increase of numbers of light-emitting dots causes a lowered yield of LED chips and a rise in manufacturing costs. To avoid this, LED chips having the small number of light-emitting dots are mass-produced and are arranged by a length being equivalent to a printing width required by an object to be printed. However, an increase in density of the optical printer head by this method leads to packaging limitation imposed by arrangements and electrical connections of the LED chips. Therefore, the LED-type optical printer head has limits to lower manufacturing costs and higher density.
Therefore, a possibility of using light-emitting devices other than the LED is examined. For example, an optical printer head using an organic EL (electroluminescence) thin-film light emitting device is disclosed in Japanese Patent Application No. Hei8-108568. In a case of the optical printer head using the organic EL thin-film light emitting device, a large number of light-emitting devices can be formed, collectively and at comparatively lower costs, on a substance with a large area and mass production of the optical printer head is made possible. Moreover, by microfabrication of an electrode of the optical printer head, the density of the optical printer head is made higher.
Generally, in the optical printer head, even if an emitting device has a small light-emitting luminance, by arranging light-emitting devices in two dimensions, it can be used for short time exposure processes. For example, in Japanese Patent Application No. Hei9-254437, an optical printer head is disclosed in which light-emitting devices are arranged in two dimensions and a picture element array with a cluster of optical fibers incorporated on a front panel is used as a printer head.
However, the optical printer head using thin-film emitting devices such as organic EL devices or a like has a problem. That is, since a maximum light emitting luminance of present organic EL devices is hundreds cd/m2 for a lifetime of scores of thousands of hours, when used for a printer head, it is difficult to satisfy both of requirements of light amounts required for exposure and practical lifetime (to be measured by required pieces of paper to be printed) at a same time. Though light-emitting operation with high luminance would be made possible by sacrificing the requirement of the lifetime and by using a replacement-type optical printer head, it is impossible to perform exact positioning among the optical printer head, photosensitive drum and optical system with an accuracy in the order of microns at a user level.
The followings are problems common to xerography-type printers:
(1) Correction is needed to a sensitivity characteristic of a photosensitive body.
(2) Correction is needed to a positional displacement of an object to be printed.
(3) Correction is needed to insufficient development that may occur in a region having small exposure amounts when printing on multiple gray scales is performed.
Since a characteristic of a surface potential of the photosensitive body does not always correspond linearly to the amount of exposure, the printer must be driven in accordance with the characteristic of a photosensitive body. Moreover, since the positional displacement of the object to be printed causes the degradation of print quality, this correction must be performed without fail. Also, since the problem of insufficient development in the region having small amounts of exposure, in general, is likely to arise in the conventional photosensitive body, some measures are necessary as in the other two cases.
Furthermore, another problem is that, when the light-emitting device is arranged in two dimensions, the number of driving circuits, wirings or like mounted outside the optical printer head inevitably increase with increases in the number of the light-emitting devices, thus interfering with higher density and miniaturization of the printer head.
In view of the above, it is an object of the present invention to provide an optical printer head with a plurality of light-emitting devices arranged in two dimensions which is capable of providing a desired amount of exposure using light-emitting devices having small light-emitting luminance, of making easy corrections to a sensitivity of a photosensitive body and to a positional displacement of an object to be printed, of performing printing on multiple gray scales and of implementing high density and miniaturization.
According to a first aspect of the present invention, there is provided an optical printer head including:
a picture element array composed of picture elements containing light-emitting devices arranged in directions of a line and a string in two dimensions;
a horizontal scanning circuit to feed data signals to each picture element string in the picture element array;
a vertical scanning circuit to sequentially select and activate each picture element line in the picture element array; and;
whereby the picture element array, horizontal scanning circuit and vertical scanning circuit are formed in a same insulating substrate.
By configuring as above, high density and miniaturization of the optical printer head are made possible. Moreover, since the exposure is repeated two or more numbers of times by a plurality of light-emitting devices arranged in a vertical scanning direction, on a same spot on a photosensitive body, even when a light-emitting device having a small amount of light to be emitted is used, desired amounts of exposure can be provided.
In the foregoing, a preferable mode is one wherein the light-emitting device is composed of organic electroluminescence devices.
Also, a preferable mode is one wherein the horizontal scanning circuit and vertical scanning circuit are composed of poly-crystal silicon thin-film transistors.
Also, a preferable mode is one that wherein includes a means for setting amounts of light to be emitted from the light-emitting device in picture elements constituting the picture element lines by each picture element line constituting the picture element array.
By configuring as above, an amount of light to be emitted by the light-emitting device of the picture element line existing on a low image density side can be larger than that of light to be emitted by the light-emitting device of the picture element line existing on a high image density side and therefore a problem that an output image becomes too white on the low image density side, can be solved.
Also, a preferable mode is one wherein the vertical scanning circuit is so operated that, in a state in which the picture element array is disposed facing a surface of a photosensitive body in a manner that a direction of the picture element line is parallel to a rotation axis of the photosensitive body, activates the picture element line containing each picture element while each picture element contained in each picture element string in the picture element array is passing sequentially on a same spot on a surface of the photosensitive body, with rotation of the photosensitive body.
Also, a preferable mode is one wherein the number of picture elements in each picture element string activated by the vertical scanning circuit is able to be changed.
By configuring as above, a surface potential of the photosensitive body can be controlled and, as a result, an amount of toner to be deposited on the surface of the photosensitive body can be changed, thus allowing multiple gray scale printing.
Also, a preferable mode is one wherein the picture elements constituting said picture element array are divided into a plurality of groups of picture elements in directions of a same line and of a same string and wherein, while the number of picture elements constituting the group of picture elements to be activated by the vertical scanning circuit is being changed, activation of the picture elements is performed for every group of the picture element of the same line.
By configuring as above, multiple gray scale printing is made possible by inputting binary data.
Furthermore, a preferable mode is one that wherein includes a detecting sensor for detecting positional deviation of insertion in a direction vertical to a direction of travelling of an object to which a toner image is transferred from the photosensitive body and a shift register for shifting data signals in the horizontal scanning circuit to correct the detected positional deviation.
By configuring as above, degradation of print quality caused by the positional deviation of insertion of the object to be printed can be corrected with accuracy.